Laser rotation rate sensor

ABSTRACT

The invention relates to a laser rotation rate sensor wherein two light beams counterrotate in a polygon equipped with reflectors at its corners, a signal depending on the rate of rotation being derived from the frequency difference. One of the reflectors employed is designed as a magneto-optic element so as to suppress lock-in. This reflector includes a prism whose base boundary face is provided with a layer of a material having a lower index of refraction than the prism material. At least one of these materials is a gyrotropic material. Almost no reflection occurs when light enters the prism at the two lateral faces due to adherence to the Brewster condition. Further, the angle of incidence of the beams on the interface with the layer having a lower index of refraction than the prism material is so large that total reflection occurs.

BACKGROUND OF THE INVENTION

The invention relates to a laser rotation rate sensor in which two lightbeams counterrotate in a polygon equipped with reflectors which dependsits corners, a signal being derived in on the rate of rotation from thefrequency difference. The device further includes, for lock-insuppression, a reflector which is designed and operated as amagnetooptic element.

It is known that laser rotation rate sensors can be used to measureinertial rotation rates in that the difference in frequency betweencounterpropagating electromagnetic waves is determined. It is furtherknown that this frequency difference disappears at input rotation ratesbelow a certain threshold value and that thus the rotation rate sensorloses its ability to measure low rotation rates. This phenomenon iscalled the lock-in effect. To avoid the lock-in effect, various measureshave been developed which, in principle, are all based on the fact thata zero frequency split is forced onto the ring laser or--in otherwords--that its operating point is placed at a point outside the lock-inband.

One of these measures is the use of the magnetooptic Kerr effect. Inthis case, a nonreciprocal (i.e. direction dependent) phase shift isforced onto the light when it is reflected at the interface between twomedia of which at least one must be gyrotropic.

Thus, a phase shift difference is generated between thecounterpropagating waves of such a rotation rate sensor and this phaseshift difference leads to the above-mentioned desired zero frequencysplit according to the following equation:

    Δν=(Δφ/2π(c/L)

where

Δν=the frequency difference;

Δφ=the phase shift difference:

c=the speed of light;

L=the length of the rotational path.

A corresponding arangement is known from U.S. Pat. No. 4,225,239. Inthat patent, a magnetooptic metal mirror is inserted in the beam path inaddition to the conventional corner mirrors and the beams impinge onthat mirror in a grazing manner.

Such a magnetic mirror, in addition to having a sufficient Kerr effect,must also have a sufficiently high reflection capability to be able toserve as a resonator mirror. Both these requirements prevent the use ofpurely metal mirrors made of ferromagnetic material which, althoughhaving a sufficient Kerr effect, do not have a reflection capabilitysufficient for the above-mentioned use (typical reflection values liebetween 40 and 70%). To overcome this drawback, U.S. Pat. No. 4,225,239teaches that the reflection capability of the pure metal surface can beincreased by applying dielectric coatings. However, this reduces theKerr effect of such a mirror to a considerable degree since, due toreflection in the dielectric layers, only a fraction of the incidentelectromagnetic wave reaches the magnetized layer.

The Kerr mirror design disclosed in German Offenlegungsschrift No. DE-OS2,432,479, which comprises an alternating sequence ofquarter-wave-length layers of a dielectric material and of aferromagnetic material, has been found to be technologically difficultto realize. To overcome this technological difficulties, GermanOffenlegungsschrift DE-OS No. 2,919,590 and U.S. Pat. No. 4,195,908teach an arrangement whereby a gyrotropic garnet layer is located infront of a dielectric layer system. However, the construction of such amirror requires provision of a plate of a nonmagnetized garnet materialhaving a gyromagnetic layer and subsequent dielectric layers applied ina suitable manner to the side of the garnet material facing away fromthe radiation source. Therefore, in spite of antireflection coatings onthe side facing the radiation, reflection losses cannot be avoided, norcan absorption losses be avoided in the garnet material itself.

A further drawback of all previously proposed Kerr mirrors is that, inorder to maintain the necessary polarization states of theelectromagnetic radiation, special additional measures must be taken inthe resonator.

Moreover, the manufacture of dielectric layer systems having highdegrees of reflection is more difficult for p polarized light, as it isused, for example, for the magnetooptic Kerr effect with transverseorientation of the magnetic field (magnetic field vector perpendicularto the plane of beam incidence) than, for example, for s polarizedlight.

The object of the invention is to provide a magnetooptic element which,by utilization of the magnetooptic Kerr effect, generates the largestpossible phase shift difference and, as a consequence thereof, thegreatest possible frequency split of the counterrotating electromagneticwaves, and which exhibits high reflectivity for the laser radiationemployed.

SUMMARY OF THE INVENTION

This problem is solved in that the reflector designed as themagnetooptic element includes a prism whose base boundary face isprovided with a layer of a material which has a smaller index ofrefraction than the prism material; that at least one of these materialsis a gyrotropic material; and that the angle of incidence of theradiation on the interface with the layer having the lower index ofrefraction is so large that total reflection occurs. The solutionaccording to the invention is preferably utilized with p polarizedlight.

Additionally, if the Brewster condition is also met, the solutionaccording to the invention has the advantage that the desiredpolarization state of the electromagnetic waves is maintained withoutfurther auxiliary means and that it is easy to manufacture.

As mentioned above, either the prism itself or the adjacent layer may bemade of gyrotropic material or both materials may be gyrotropic; themagnetic field acts on this material so that it produces themagnetooptic Kerr effect.

In contradistinction to the arrangement disclosed in the above-mentionedU.S. Pat. No. 4,225,239, the reflector according to the invention isused as a corner reflector in triangular or square circulation paths andnot as an additional reflector. The use of prisms instead of mirrors asreflectors is known per se (U.S. Pat. No. 3,545,866). The significanceof the invention, however, is that such a prism is incorporated in amagnetooptic element while utilizing the effect of weakened totalreflection.

If the layer disposed at the base boundary face of the prism is made ofa gyrotropic material, the requirement that a gyrotropic material beused which has an index of refraction that is much lower than that ofthe prism material, constitutes a limitation of the number of substancesthat can be employed, since generally most gyrotropic substances have ahigh index of refraction. This makes it difficult, to say the least,locate a substance that also has the greatest possible Faraday rotationand the lowest possible absorption.

Embodiments of the invention are therefore directed to modification ofthe proposed solution to the extent that the significance of thisrequirement for a certain index of refraction of the gyrotropic materialbecomes less important and thus the gyrotropic material can be selectedprimarily with regard to the other requirements.

In one embodiment the layer bordering the base boundary face of theprism is designed as a multiple layer. On the one hand, the partiallayer directly adjacent the base boundary face of the prism is made ofgyrotropic material, but the index of refraction of this layer is equalto or greater than that of the prism and, moreover, it is followed by asecond layer of dielectric material which has an index of refractionlower than the prism. The prism aperture is equal to the index ofrefraction of the prism multiplied by the sine of the angle of incidenceof the light impinging on the prism. In this case, total reflectionoccurs at the interface between the gyrotropic layer and the dielectriclayer with the light penetrating the gyrotropic material which now haslow absorption. Thus it is possible to influence the phase of the lightby means of the magnetic field.

The second layer may also be air or a vacuum.

In another embodiment, the base face of the prism borders at adielectric layer having a lower index of refraction. This layer restsagainst the second layer of gyrotropic material into which, if thethickness of the first layer is selected in a suitable manner, asufficient portion of the transversely attenuated wave generated duringtotal reflection at the interface between the prism and the first layercan enter. With suitable magnetization of the gyrotropic layer, thisthen results in the desired phase influence.

If, in this case, the index of refraction of the gyrotropic material isless than the prism aperture, the light will likewise propagate as atransversely attenuated wave in the gyrotropic material. To maximizethis phase shift difference, the gyrotropic layer should be thicker thanthe penetration depth of the transversely attenuated wave propagatingtherein. This penetration depth is about one to two wavelengths of thelight employed.

If, however, the index of refraction of the gyrotropic material isgreater than the prism aperture, then the thickness of the gyrotropiclayer must be optimized in such a manner that a maximum phase shiftdifference is realized. A further increase in the phase shift differencecan be realized in this case in that a third layer of dielectricmaterial follows the gyrotropic layer. Such a double-sided coverage ofthe gyrotropic layer with dielectric layers can now be accomplished inthat the Kerr effects leading to the phase shift difference mutuallyreinforce themselves during reflection at the upper and lower boundaryfaces of the gyrotropic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, shows a laser rotation rate sensor including a reflectoraccording to the invention with a single layer at the base boundary faceof the prism; and

FIGS. 2-4, other embodiments of the reflector 5 of the rotation ratesensor of FIG. 1 with multiple layers.

The rotation rate sensor shown in FIG. 1 includes an optical amplifier1, a mirror 2, a partially transmissive mirror 3, a further mirror 4, abeam divider 4a, a reflector 5 and an optical detector 7 for measuringthe frequency difference of the waves. The counterrotating laser beamsare designated 8 and 9. Reflectors 2, 3 and 5 are designed and arrangedin such a manner that the illustrated rotary paths are created. Beam 9is reflected at mirror 2, partially reflected by mirror 3 and theremaining portion of the beam is returned by reflector 5 to opticalamplifier 1. The portion passing through mirror 3 reaches detector 7.

Beam 8 is deflected in reflector 5, is partially deflected to mirror 2by the partially transmissive mirror 3 and is returned from mirror 2 tothe optical amplifier. The portion passing through the partiallytransmissive mirror is directed by mirror 4 onto beam divider 4a and apart thereof is likewise deflected toward detector 7.

In order to suppress the lock-in effect, reflector 5 is designed in aspecial manner. It includes a prism 5a (e.g. made of strontium titanate)having a certain index of refraction n₁ ; the lateral faces 5b of theprism are inclined in such a manner that, due to meeting the Brewstercondition, the generated p polarized beams (8 or 9, respectively) arenot reflected. A layer 5c of a gyrotropic material, e.g. ferrimagneticgarnet material, is applied to the base face 5d of the prism; thismaterial has an index of refraction n₂ which is less than the aperturein the prism 5a. Thus, there will occur a weakened total reflection ofthe beams at the interface 5d between prism 5a and layer 5c. A smallportion of the light enters the gyrotropic layer to a slight depth.Since, as a consequence of this, and due to the applied magnetic field(perpendicular to the plane of the drawing and not shown) there occurs aKerr effect, the beams (9 and 8) experience different shifts in phase.This makes it possible to sense even low rotational rates of thearrangement.

FIG. 2 shows a differently designed reflector which may replacereflector 5 of FIG. 1. This reflector includes the coupling prism 11 ofdielectric material (index of refraction n₁) and the angle β, thegyrotropic layer 12 (index of refraction n₂) applied thereonto, and anadditional dielectric layer 13 (index of refraction n₃).

The laser beams 14 impinge on the lateral faces of the coupling prism 11(made e.g. of glass) at the Brewster angle α_(Br) and, due to thegreater density (n₁) of the prism material, are refracted toward theperpendicular. They impinge on layer 12 (made, for example, offerromagnetic garnet material) at the angle α₁ and, since the index ofrefraction (n₂) of the gyrotropic material is greater there, arerefracted again toward the perpendicular. Thereafter, they impinge onlayer 13 at the angle α₂ and, since the material (e.g. cryolite, MgF₂)of this dielectric layer has a clearly smaller index of refraction (n3)(e.g. close to 1.0), total reflection occurs at the interface betweenlayers 12 and 13. The magnetization of layer 12 produces a nonreciprocalphase influence on the counterpropagating waves and thus the desiredphase shift difference.

An index of refraction of close to one is given above for the dielectriclayer. It should be noted in this connection that layer 13 may also beambient air or an ambient vacuum for which the condition of the index ofrefraction being close to one or equal to one applies. However, adistinct layer of a solid dielectric material may also be utilized. Thethickness of layer 12 of gyrotropic material must be selected in such amanner that the Kerr effects leading to the phase shift differenceduring reflection at the upper and lower interfaces of the gyrotropiclayer 12 mutually reinforce themselves. For optimization, care must betaken that the absorption losses of reflector 5 increase monotonicallywith the thickness d₂ of layer 12 and the resulting phase shiftdifference is periodic in the thickness of layer 12.

The following comments apply to the above-mentioned angles andrefractive indices:

    α.sub.Br =arc tan n.sub.1

    β=α.sub.0 +α.sub.Br

    2α.sub.0 =beam deflection angle effected by reflector 5 (e.g. 60°)

    α.sub.1 =β-arc sin (sin α.sub.Br /n.sub.1)

    α.sub.2 =arc sin (n.sub.1 sin α.sub.1 /n.sub.2)

    n.sub.3 <n.sub.1 ·sin α.sub.1 =prism aperture

In the embodiment of FIG. 3, the light beam 15 enters prism 16 at theBrewster angle α_(Br) so as to avoid reflection losses. Prism 16 is madeof commercially available glasses having a suitable index of refraction,such as, for example, boron crown glass, heavy flint, strontiumtitanate, zirconium dioxide, quartz or the like. To avoid reflectionlosses, the entrance faces of the prism may also be demirrored. The beamimpinges on the interface 17 with the dielectric layer 18 where it istotally reflected--if the material of layer 18 has a sufficiently lowindex of refraction--and is deflected by the desired angle. Thetransversely attenuated wave generated during total reflectionpropagates along interface 17. Its field intensity in layer 18 decreasesexponentially with the distance from interface 17, but--if the thicknessof layer 18 is selected to be suitably small (in the order of magnitudeof 10 nm)--still extends into gyrotropic layer 19. The thickness of thegyrotropic layer 19 must here be selected in accordance with the ratioof its index of refraction to the prism aperture. With suitablemagnetization of medium 19, a nonreciprocal phase influence on the partof the counterpropagating waves generates the desired phase shiftdifference.

The embodiment of FIG. 4 includes three layers. Here again, light beam24 enters prism 20 at the Brewster angle α_(Br). The index of refractionof the dielectric layer adjacent the prism base is selected to be sosmall that total reflection occurs at the interface between prism 20 anddielectric layer 21. The gyrotropic layer 22 is followed by a furtherdielectric layer 23. The structure of layers 21, 22, 23 with respect tothickness and index of refraction is selected in such a manner that theKerr effects occurring due to the magnetic field mutually reinforce oneanother during reflection at the upper and lower interfaces of thegyrotropic layer.

We claim:
 1. A laser rotation rate sensor wherein two light beamscounterrotate in a polygon equipped with reflectors in its corners, asignal being derived from the frequency difference which depends on therate of rotation, said rotation rate sensor further including areflector comprising a magneto-optic element for the suppression oflock-in, characterized in that said reflector includes a prism whosebase boundary face is provided with a layer of a material having a lowerindex of refraction than the prism material; that at least one of saidmaterials is a gyrotropic material; and that the angle of incidence ofthe beams on the interface with respect to the layer having the lowerindex of refraction is so large that total reflection occurs.
 2. A laserrotation rate sensor according to claim 1, characterized in that thelayer adjacent the base boundary face of the prism is a double layer;the directly adjacent portion of the double layer is made of adielectric material, having an index of refraction which is so muchlower than that of the prism material that total reflection occurs atthe interface toward the dielectric layer; that this dielectric layer isfollowed by a layer of gyrotropic material and that the thickness of thedielectric layer is selected to be so small that the transverselyattenuated wave generated during total reflection, although passing intothe layer of gyrotropic material, does encounter the necessaryreflectivity.
 3. A laser rotation rate sensor according to claim 2,characterized in that the index of refraction of the gyrotropic materialis less than that of the prism aperture.
 4. A laser rotation rate sensoraccording to claim 3, characterized in that the thickness of thegyrotropic layer is greater than the penetration depth of thetransversely attenuated wave.
 5. A laser rotation rate sensor accordingto claim 1, characterized in that the layer adjacent the base boundaryface of the prism is a triple layer; that the directly adjacent layerportion of the triple layer is made of a dielectric material having anindex of refraction that is so much lower than that of the prismmaterial that total reflection occurs at the interface with thedielectric layer; that this first layer of dielectric material isfollowed by a layer of a gyrotropic material; that the thickness of thefirst layer is selected in such a manner that the transverselyattenuated wave generated during total reflection still passes into thelayer of gyrotropic material but does encounter the necessaryreflectivity and that this layer of gyrotropic material is followed by afurther layer of dielectric material.
 6. A laser rotation rate sensoraccording to claim 5, charcterized in that the index of refraction ofthe gyrotropic material is greater than that of the prism aperture (20).7. A laser rotation rate sensor according to claim 1, characterized inthat, due to adherence to the Brewster condition, almost no reflectionoccurs when light enters the prism at the two lateral faces 5(b).
 8. Alaser rotation rate sensor according to claim 2, characterized in that,due to adherence to the Brewster condition, almost no reflection occurswhen light enters the prism at the two lateral faces.
 9. A laserrotation rate sensor according to claim 3, characterized in that, due toadherence to the Brewster condition, almost no reflection occurs whenlight enters the prism at the two lateral faces.
 10. A laser rotationrate sensor according to claim 4, characterized in that, due toadherence to the Brewster condition, almost no reflection occurs whenlight enters the prism at the two lateral faces.
 11. A laser rotationrate sensor according to claim 5, characterized in that, due toadherence to the Brewster condition, almost no reflection occurs whenlight enters the prism at the two lateral faces.
 12. A laser rotationrate sensor according to claim 6, characterized in that, due toadherence to the Brewster condition, almost no reflection occurs whenlight enters the prism at the two lateral faces.
 13. A laser rotationrate sensor, comprising:an optical amplifier having first and secondopposite ends for emitting first and second beams of light, said firstand second beams of light travelling in opposite directions around aclosed optical path; an optical detector; a magneto-optic reflectorpositioned in the path of said first laser beam, said reflectorincludinga prism having a given index of refraction, said prism havingat least first and second surfaces, and a gyrotropic layer having firstand second opposing surfaces, said first surface being affixed to thesecond surface of said prism, the index of refraction of said gyrotropiclayer being less than the index of refraction of said prism, said firstlaser beam impinging at the Brewster angle on the first surface of saidprism, being totally reflected at the second surface of said prism andbeing emitted as an output beam from said reflector, a magnetic fieldbeing impressed on said magnetooptic reflector in a directionperpendicular to the first surface of said gyrotropic layer and to theplane of said first laser beam; and optical means transmitting theoutput beam from said reflector and said second laser beam to saidoptical detector, the output of said optical detector corresponding tothe difference in frequency between said first and second beams oflight.
 14. A laser rotation rate sensor according to claim 13, whichfurther comprises a dielectric layer interposed between said prism andthe first surface of said gyrotropic layer, the index of refraction ofsaid dielectric layer being less than the index of refraction of saidprism.
 15. A laser rotation rate sensor according to claim 14, whichfurther comprises a second dielectric layer affixed to the secondsurface of said gyrotropic layer.